The present invention relates to a switched capacitor input stage for an imaging front end. In particular, it relates to a circuit that can amplify signals from an image sensor and input the amplified signal into an analog front end circuit.
Analog front-ends (AFEs) for imaging systems have to interface with image sensors. In many applications, the image sensor output is capacitively coupled to the AFE input. The coupling capacitor can provide a voltage level-shift between the sensor and the AFE. This is particularly useful for coupling certain image sensors, such as CCD sensors, which might have a DC output voltage different from the allowable input compliance of the AFE.
The desired charge on the coupling capacitor must be established by performing, by the AFE, what is sometimes called a “DC restore”. A DC restore circuit commonly includes a switch and a voltage source to recharge the coupling capacitor. The DC restore function can occur at various times depending on the image sensor. The DC restore circuit may only be active for certain pixels, for example 10-20, out of the 10,000 or more pixels in the image sensor pixel array. When the DC restore circuit is inactive, leakage currents off of the coupling capacitor can change the voltage at the capacitor. The leakage currents are often into the inputs of the AFE, and can cause image artifacts.
Typically, DC restore operations are performed when an output signal from the imaging sensor is maintained at a known value and the output signal does not carry image information. During pixel read out operations, image sensors conventionally perform precharge operations and other processes during which the sensor outputs settle to some stable reset or reference level, and AFE circuits often use this stable reference level to perform the DC restore operation. Image sensors are now being designed that do not provide a stable reference level during every pixel, therefore, a new interface between an image sensor and an AFE circuit is required.